The present invention relates to a two-piece piston, also known as articulated piston, for use in internal combustion engines. More particularly, the invention deals with a method for manufacturing the upper portion, or head, of an articulated piston as well as the piston obtained thereby.
The current trend points to an increasing use of articulated pistons in internal combustion engines, especially in the modern diesel engines which run at high speeds and deliver a high specific output, at the same time that their design is addressed to meet such requirements as low weight, low fuel and lubricating oil consumption, and lower noise and emission levels, these latter features the object of more and more stringent laws in many countries.
In an articulated piston the upper portion, or head, and the lower portion, or skirt, perform independent functions. The skirt is mounted by means of a wrist pin on a pair of pin bosses located at the lower end of members depending from and integral with the head. A gap, or clearance, is maintained between the head lower end and the skirt upper end so that with the piston fitted in the engine there is no contact between the head and the skirt. This prevents the flow of heat from the head to the skirt, thereby allowing the skirt to be mounted with very small skirt-to-cylinder clearances, which brings about a reduction of noise. Moreover, the lateral, or rocking, motion of the articulated piston caused by the connecting rod is reduced. Therefore, that portion of the head provided with the ring grooves can be fitted with small clearances, thereby improving the piston ring stability. This in turn will reduce lubricating oil consumption and minimize blowby.
Owing to the high temperatures generated on the piston top during the combustion process in diesel engines, it is necessary to remove part of the heat from said portion of the piston, especially from the combustion bowl, in order to prevent the formation of thermal cracks which may cause fracture of the top. One well known solution for this problem is to provide the piston with a chamber in which cooling oil circulates. In the articulated piston this chamber is composed of a circumferential recess, or groove, on the head and a tray on the top of the skirt. The cooling medium is supplied to the chamber by a nozzle located at the engine crankcase. The reciprocating travel of the piston causes the cooling oil to splash against the chamber wall, thereby removing part of the heat from the top, especially around the combustion bowl.
There is a basic technical principle of diesel engines under which the center of the combustion bowl on the piston top must be in alignment with the fuel injection nozzle so as to meet combustion efficiency requirements. Thus, the location of the fuel injection nozzle in certain engine designs requires that the combustion bowl be offset from the piston top centerline. In such cases, it is necessary to design the cooling chamber as to ensure an effective cooling, i.e., the thickness of the chamber inner wall, which separates the chamber from the combustion bowl, must be as reduced and uniform as possible. This requires the cooling chamber to be asymmetric in relation to the piston longitudinal axis.
In conventional articulated piston heads, in which the top and pin boss portions are an integral part, the provision of an asymmetric groove on the lower portion is possible only when the part is obtained by casting. However, the casting method has proved inappropriate for making heads having structural properties consistent with the loads imposed on them. For this reason, at present there is a clear preference for the forging method. This method, however, also has a drawback. That is, it is not possible to provide the top lower portion with a groove during the forging operation. Thus, the groove must be made by machining, and then only if it is an axisymmetric chamber. An asymmetric chamber would be unfeasible due to process limitations, as it is impossible for the machining tool to access the region to be machined.
The present invention relates to a piston in which the parts composing the head of an articulated piston namely, the top portion and the pin boss portion, are made separately and then joined together by welding. Said concept is disclosed in Brazilian patent application 8805716.